1. Field of the Invention
The present invention relates to a method for dewatering water-containing particle collections in which at least one of a possible multiple of dewatering stages is effected with the aid of gas to which a pressure difference is imparted through and across the particle collection.
2. DESCRIPTION OF THE PRIOR ART
Water-containing particle collections, such as mineral suspensions and fibre suspensions, are often dewatered by first mechanically pumping the particle collection onto a filtering medium and/or by pressing said collection against said medium under high pressure, such as to form a filter cake from which water is then removed under the influence of gas under pressure. Dewatering can either be accomplished by employing a vacuum and/or by using a pressurized gas, said gas normally being compressed air. Thus, in both instances, and also in different combinations thereof, the filter cake is subjected to a pressure difference which consists, in principle, of the sum of the low pressure on the suction side and the high pressure on the pressure side. The dewatering process is effected through the influence of this pressure difference in three successive phases, mainly a first penetration phase, which is commenced by subjecting fully saturated filter cake to pressurized gas on one side thereof, and is terminated when the gas breaks through the filter cake (penetration) and thus exits from the other side of said cake. During this phase, water present in the cake is simply displaced forwardly by the advancing gas front. During the second phase, the drainage phase, the gas flows through the filter cake while maintaining a pressure difference within the cake, the water in this case being forced to move in the same direction as the gas. This phase will terminate when the water capillary forces are in equilibrium with the gas pressure prevailing locally within the filter cake, with a subsequent decrease in water flow. When this state is reached, further water can only be removed by vaporizing water into the passing gas flow. This last dewatering phase, the drying phase, is thus dependent on gas mass flow, temperature and pressure.
The aforedescribed dewatering mechanism is employed in several processes that are applied industrially for the purpose of dewatering particle collections in three phases or stages. Thus, a common feature of these processes is that dewatering is effected in at least one of the process stages with the aid of a gas to which there is imparted a pressure difference through and across the particle collection.
For instance, "Solid/Liquid Separation Technology", published by Uplands Press Ltd, 1981, describes on page 331 one such method in which gas is blown into the particle collection at a pressure of 6 bars, whereas the final stage is effected by mechanical pressing-dewatering of the particle collection.
According to another known method, described in Swedish patent application SE-8603070-7 (Sala International), the final lowering of the moisture content of the particle collection is achieved by compressed-air blowing. The above-cited publications and patent applications are incorporated herein by reference as if the texts thereof were fully set forth herein.
A common feature of all methods known hitherto, however, is that further lowering of the moisture content can only be achieved by extending the blowing time or the cake-heating time. Both cases thus require a high energy input. Furthermore, an extended blow time will lower the capacity of the filter medium used.
Because of higher energy costs and the ever greater industrial demands for more economical particle suspension dewatering processes, there is a need for further development of such dewatering processes which will render such processes less prohibitive, from the aspect of energy consumption, in the handling of these materials. In present times, energy costs are considered a very important factor in the total dewatering costs. Consequently, there is found an ever increasing desire for particle-collection dewatering processes which fulfill the requirements of lower energy consumption and/or higher productivity.